The present invention relates to an optical disk for recording information on it by employing a reversible phase change in accordance with laser beam irradiation.
Since the optical disk recording scheme employing a laser beam enables large-capacity recording and high-speed access in the non-contact manner, it has been put into practical use in the form of a large-capacity memory. Optical disks are classified into ROM optical disks (known as compact disks and laser disks), write-once optical disks (on which the user can record information), and re-writable optical disks (on and from which the user can repeatedly record and erase information). Write-once and re-writable optical disks will soon be used as external memories for computers, or for document and image files.
Re-writable optical disks include a phase change type optical disk (that utilizes the phase change of a recording film) and a magneto-optical disk (that utilizes changes in magnetization direction of a perpendicular magnetized film). Of these two types of disks, the phase change type optical disk is expected to be popular as a re-writable optical disk because (1)it does not require an external magnetic field, and (2) can easily perform an overwrite operation.
Conventionally, in the phase change type optical disk, a write access using a recording film that undergoes a phase change between the crystalline phase and the amorphous phase is performed in accordance with laser beam irradiation. A laser beam spot having a high power, in accordance with information to be recorded on the recording film, is irradiated on the phase change type optical disk to locally increase the temperature of the recording film. Recording is thus performed by causing a phase change between the crystalline phase and the amorphous phase. A change in the optical constant accompanying this phase change is read by a low-power laser beam as a difference in intensity of the reflected light, thereby reproducing the information.
For example, when a phase change type optical disk using a recording film has a comparatively long crystallization time the disk is rotated,and a laser beam is irradiated on the recording film of the disk to heat the recording film to a temperature equal to or higher than its melting temperature. After the laser beam passes, the disk is quickly cooled to set this portion in the amorphous state, thereby performing recording. During erasing, the recording film is exposed to temperatures in a crystallization enable temperature range between a crystallization temperature and melting temperature for a sufficiently long period of time required for promoting crystallization, so that the recording film is crystallized. To accomplish this, a know method of irradiating a laser beam having an elliptic spot in the laser beam traveling direction of the disk is used. To perform a pseudo-overwrite operation with two beams in which new information is recorded while erasing already recorded data, an erasing laser beam having an elliptic spot is irradiated upon the recording film preceding a recording laser beam having a circular spot.
When a disk using a phase change recording film capable of high-speed crystallization is employed, a single laser beam focused to have a circular spot is used. According to a conventional method, a phase change between the crystalline and amorphous states is caused by changing the power of the laser beam between two levels. More specifically, when a laser beam having a power that can heat the recording film to a temperature equal or greater than to the melting temperature or more is irradiated on the recording film, most of the recording film is set in the amorphous state upon being cooled. Meanwhile, when a laser beam having a power that heats the recording film to a temperature of greater than or equal to the crystallization temperature, but less than or equal to the melting temperature is irradiated on the recording film, most of the recording film is set in the crystalline state.
To form the recording film of a phase change type optical disk, GeSbTe, InSbTe, InSe, InTe, AsTeGe, TeO.sub.x --GeSn, TeSeSn, SbSeBi, or BiSeGe belonging to a chalcogenide-based material is employed. Material is deposited in accordance with resistance heating vacuum deposition, electron beam vacuum deposition, sputtering, or the like. The recording film, immediately after deposition, is set in a kind of amorphous state. The entire recording film is set in the crystalline state by initialization so that an amorphous recorded portion is formed upon recording information on the recording film. Recording is achieved by forming an amorphous portion in the recording film which is in the crystalline state.
As described above, conventionally, in a phase change type optical disk, a difference in light amount between the light reflected by the recorded portion and that reflected by the erased portion is mainly detected as a reproduced signal. Generally, however, when the amount of light reflected by the recorded portion differs from that reflected by the erased portion, the absorption of the recorded portion differs from that of the erased portion. In an overwrite operation, a recording mark distortion caused by this absorption difference occurs.
In order to eliminate this recording mark distortion a phase change type optical disk, in which heat energies required for melting the recording mark portion in recording laser irradiation and the erased portion are set equal by controlling the absorption difference is very effective.